1. Field of the Invention
This invention relates to an improved method for manufacturing micromechanical devices, more particularly to manufacturing these devices such that their moving parts do not stick together.
2. Background of the Invention
Micromechanical devices are very small devices such as motors, gears, light modulator cells, etc., that are manufactured out of a semiconductor substrate, typically by a repeated process of masking and etching. They are activated by circuitry implanted into, or laid down upon, the substrate. As electronics move towards more and more power in a smaller and smaller space, micromechanical devices become more important.
One problem with these devices is the wear and tear they undergo during operation. A typical silicon micromotor loses its rotor after only about ten thousand revolutions. The silicon just wears away. The erosion is caused by the rotor sticking to surrounding structures, including its own axle, as it revolves, eating away the silicon. Another problem can occur when, instead of the rotor being worn away, it just sticks such that it will no longer move. Various methods of laying down protective layers exist, but each have their problems.
Some manufacturing processes use a deposited film on the devices after they are complete. Others try to work the layer into the existing process, requiring expensive and non-standard processing. Obviously, some method of preventing these devices from sticking together is needed.
An additional problem with micromechanical devices is the size of the activation circuitry. For example, micromechanical light modulators, such as the digital micromirror device, sometimes require activation circuitry of four electrodes for each 17 .mu..sup.2 mirror. The mirror is suspended above its electrodes by hinges which in turn are supported by posts. Using the axis of the hinges as the center line of the mirror, each mirror has two electrodes on either side of its center line, directly under the mirror on the substrate. One electrode on each side is the addressing electrode, the other is the landing electrode, with the addressing electrodes adjacent to the center.
The landing electrodes and the mirror are held at the same bias voltage, preventing shorts or current flow when the mirrors touch their landing electrodes. In order to cause the mirror to deflect and touch the landing electrode, the address electrodes are set at a different bias. Because the area on the substrate within which these electrodes must reside is so close, it is relatively common for a particle to rest such that it touches both the landing electrodes and the address electrodes. Since they are held at different voltage levels, this can cause shorts in the activation circuitry of the modulator, rendering it unusable. Other micromechanical devices can suffer from these same problems, whenever the driving circuitry is close enough to allow particles or debris to cause shorts.
Therefore, a method is needed that allows micromechanical devices to run for longer period of time without sticking together, thereby causing wear. Additionally, it would be beneficial if the method preventing particulates and debris from damaging the circuitry which activates the micromechanical devices.